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Cryptography

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CRYPTOGRAPHY PPTs by Saroj Sarang Date

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  • how the cipher text is converted to plaintext without knowing the key
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Cryptography

  1. 1. <ul><li>CRYPTOGRAPHY </li></ul><ul><ul><li>Ms. Date Saroj </li></ul></ul><ul><ul><li>ME CSE (Part Time ) </li></ul></ul>
  2. 2. <ul><li>CRYPTOGRAPHY comes from the Greek words for “ secret writing”. </li></ul><ul><li>It is the science of devising methods that allow information to be sent in a secure form in such a way that the only person, who possess the key can read it. </li></ul><ul><li>It is the collection of algorithm & associated procedures for hiding & unhiding information </li></ul>
  3. 3. <ul><li>Plaintext :Messages to be encrypted. </li></ul><ul><li>Cipher text: Output of encryption process. </li></ul><ul><li>Key </li></ul><ul><li>Intruder( Active & Passive) </li></ul><ul><li>Channel </li></ul>
  4. 4. Encryption Model, E Channel Decryption Model, D Introduction to Cryptography Ek DK Intruder p L A I N T E XT P L A I N T E X T CIPHERTEXT Active Intruder Passive Intruder FIG. THE ENCRYPTION MODEL (For a Symmetric-Key Cipher)
  5. 5. <ul><li>The messages to be encrypted ,known as Plaintext , are transformed by a key. </li></ul><ul><li>The output of encryption process, known as Cipher text , is the transmitted through the channel . </li></ul><ul><li>We assume that the intruder hears & accurately copies down the complete Cipher text. However, unlike the intended receiver, the intruder does not know what the decryption key is and so can’t decrypt the Cipher text easily. </li></ul>Cryptography
  6. 6. Cryptography <ul><li>Passive intruders can only listen to the communication channel </li></ul><ul><li>Active intruders can modify the legitimate messages before they get to the receiver. </li></ul><ul><li>Cryptanalysis: The art of breaking ciphers. </li></ul><ul><li>Cryptography: The art of devising ciphers. </li></ul><ul><li>& Cryptology means ----------- </li></ul>CRYPTOLOGY CRYPTANALYSIS CRYPTOGRAPHY = +
  7. 7. <ul><li>It will be often useful to have a notation for relating plaintext, cipher text & Keys. </li></ul><ul><li>Let’s assume that ,the plaintext has n characters , drawn from an alphabet X. </li></ul><ul><li>We represent the plaintext msg. as a vector of n symbols— X={x1,x2,x3,……xn} </li></ul><ul><li>Before transmitting, the sender Encrypts the msg. </li></ul><ul><li>This is done by a rule that maps X into a sequence of new characters called as Cipher text. </li></ul><ul><li>Y={y1,y2,y3,…..,yn} </li></ul>Notation:
  8. 8. <ul><li>The mapping from X to Y is based on an encryption rule ,which we may write as </li></ul><ul><li>Y=E k (X) </li></ul><ul><li>The subscript k (Key) denotes a particular rule, employed in a ciphersystem to determine the cipher text. </li></ul><ul><li>The cipher text msg. is transmitted to the receiver . </li></ul><ul><li>In order to read the original msg.,the receiver must use another rule ,called a decryption rule . </li></ul>Notation: Contd…..
  9. 9. <ul><li>This decryption rule can be looked as an inverse mapping defined by encryption rule. </li></ul><ul><li>X=Dk (Y) </li></ul>Notation: Contd……. Y = Ek (X) X = Dk (Y) X = Dk(Ek(X))
  10. 10. <ul><li>Let’s look at the methods by which cryptosystems can be attacked: </li></ul><ul><li>1. Cipher text-only attack </li></ul><ul><li>2. Known plaintext attack. </li></ul><ul><li>3. Chosen plaintext attack </li></ul><ul><li>4. Chosen cipher text attack. </li></ul><ul><li>Regardless of any attack intruder’s task is to determine the key. If the key is known, the cryptosystem is broken. </li></ul>
  11. 11. <ul><li>Encryptions methods have historically been divided into two categories: </li></ul><ul><li>1. Substitution Ciphers </li></ul><ul><li>2.Transposition Ciphers </li></ul>
  12. 12. <ul><li>In a substitution cipher each letter or group of letters is replaced by another letter or group of letters. </li></ul><ul><li>One of the oldest known ciphers is the Caesar Cipher . In this method a becomes D, b becomes E,….. , z becomes C. </li></ul><ul><li>For example attack becomes DWWDFN. </li></ul><ul><li>A generalization is shift cipher text alphabets by k letters. </li></ul>
  13. 13. Transposition Ciphers <ul><li>Substitution ciphers preserves the order of alphabets. </li></ul><ul><li>Transposition ciphers reorder the letters . </li></ul>
  14. 14. Some simple cipher systems <ul><li>1. The shift Cipher </li></ul><ul><li>2. The Vigenere Cipher </li></ul><ul><li>3. The permutation Cipher </li></ul><ul><li>4. The Substitution Cipher </li></ul>
  15. 15. Codes <ul><li>It is also possible to increase the security of encryption by encrypting the entire words, phrases, sentences, before the applying to the encryption model. </li></ul><ul><li>Secret codebook is shared between sender & receiver. </li></ul><ul><li>Even if the cipher has been broken , Intruder will still face the problem of decoding it. </li></ul><ul><li>Provides data compression to the plaintext. </li></ul>
  16. 16. Codes contd…. <ul><li>The combination of coding & ciphering, if used properly, can produce a cryptosystem that is far more difficult to break than a cryptosystem that uses either. </li></ul><ul><li>DISADVANTAGE </li></ul><ul><li>To prevent from the attacks, the codebook needs to be changed frequently. But changing & securely communicating codebooks is difficult challenge. So, codes are not well adapted in modern communication systems. </li></ul>
  17. 17. <ul><li>THANK YOU !........ </li></ul>

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